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[Tim Gardner <tim.gardner@xxxxxxxxxxxxx>]

Came in via v4.4.48

** Changed in: linux (Ubuntu Xenial)
       Status: In Progress => Fix Released

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https://bugs.launchpad.net/bugs/1662673

Title:
  systemd-udevd hung in blk_mq_freeze_queue_wait testing unpartitioned
  NVMe drive

Status in linux package in Ubuntu:
  Fix Released
Status in linux source package in Xenial:
  Fix Released
Status in linux source package in Yakkety:
  Fix Committed
Status in linux source package in Zesty:
  Fix Released

Bug description:
  For reference, here is the stack of systemd-udevd seen in the hang:

  [ 1558.214013] INFO: task systemd-udevd:1778 blocked for more than 120 seconds.
  [ 1558.214318] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
  [ 1558.214556] systemd-udevd   D 00003fff8dbdf7a0     0  1778      1 0x00040000
  [ 1558.214637] Call Trace:
  [ 1558.214673] [c000000004ad3790] [c0000000007aac20] schedule_timeout+0x180/0x2f0 (unreliable)
  [ 1558.214779] [c000000004ad3960] [c0000000000158d0] __switch_to+0x200/0x350
  [ 1558.214870] [c000000004ad39c0] [c0000000007adbb4] __schedule+0x414/0x9e0
  [ 1558.214961] [c000000004ad3a90] [c0000000003b4e54] blk_mq_freeze_queue_wait+0x64/0xd0
  [ 1558.215107] [c000000004ad3af0] [d000000034011964] nvme_revalidate_disk+0xd4/0x3a0 [nvme]
  [ 1558.215386] [c000000004ad3b90] [c0000000003c2398] rescan_partitions+0x98/0x390
  [ 1558.215508] [c000000004ad3c60] [c0000000003bb7ac] __blkdev_reread_part+0x9c/0xd0
  [ 1558.215599] [c000000004ad3c90] [c0000000003bb818] blkdev_reread_part+0x38/0x70
  [ 1558.215935] [c000000004ad3cc0] [c0000000003bc334] blkdev_ioctl+0x3b4/0xb80
  [ 1558.216016] [c000000004ad3d20] [c0000000002cbcd0] block_ioctl+0x70/0x90
  [ 1558.216114] [c000000004ad3d40] [c000000000296b38] do_vfs_ioctl+0x458/0x740
  [ 1558.216192] [c000000004ad3dd0] [c000000000296ee4] SyS_ioctl+0xc4/0xe0
  [ 1558.216275] [c000000004ad3e30] [c00000000000a17c] system_call+0x38/0xb4

  It appears that systemd-udevd is triggering every time HTX writes to
  the boot sector (partition table) of the raw drive, and this is
  causing the revalidate calls which expose the issue with the block
  driver mq freeze. With a partition table on each drive, HTX will no
  longer be writing the partition table and no longer triggering systemd
  to re-read the partition table and try to freeze I/O.

  The fix for this is provided by the following upstream commit:

  966d2b0 percpu-refcount: fix reference leak during percpu-atomic
  transition

  which needs to be pulled into 16.04 (as well as newer releases).

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